Nowadays, in the field of pharmaceuticals, nanotechnology is widely applied in the research on medicine prophylaxis, diagnosis, drugs delivery and treatment of diseases. Nanoparticles are useful as carriers for drug delivery. They have many advantages, such as enhancing the stability of drugs in the gastrointestinal tract, improving the absorption and bioavailability of orally administered drugs, lowering the dosage of drugs so that the side effects caused by high dosage can be reduced, and administering drugs specifically. However, due to safety concerns, factors such as the source of materials and the reagents used tend to limit the applications of nanoparticles.
Chitosan is a drug carrier currently regarded as having extremely high safety (LD50>4 g/kg). Generally, chitosan is processed and made into microspheres. For example, CN1698900 discloses a process for preparing chitosan drug carrying microspheres which is characterized by dissolving a hydrophilic medicament in an acetate-sodium acetate buffer solution to form an aqueous solution, pressing the aqueous phase into an oil phase through a microporous membrane with pressure to form milk-like droplets with a uniform dimension, and subjecting the droplets into a two-step hardening process so that they become a cross-linking solid form. The above-mentioned “two-step hardening process” comprises the step of adding an ionic gelatin agent (e.g., triphenyl phosphate (TPP)) to make the chitosan particles aggregate through their molecular chains, and that of adding a chemical cross-linking agent to make them become a cross-linking solid form. However, the method mentioned above cannot obtain the chitosan particles of nano scale.
Due to the formation of strong hydrogen bonds between chitosan and its adjacent molecules and organic acids (e.g., lactic acid), chitosan is hardly dissoluble in water. The solubility of chitosan in water becomes an important factor that influences chitosan's application. It is known that the water solubility of chitosan can be controlled by conducting an acetylation between chitosan and an acid anhydride under certain reaction conditions in organic phase to produce chitosan with different acetylation levels. Moreover, the preparation of chitosan nanoparticles is suggested.
US 2005/0226938 A1 discloses a method for preparing cross-linking core and core-shell nanoparticles polymers from chitosan which comprises reacting chitosan and at least one carboxylic acid having at least two carboxyl groups. Preferably, the activator used is carbodiimide.
US 2006/0013885 A1 discloses water-soluble chitosan nanoparticles for delivering an anticancer agent and a preparing method thereof. The method comprises linking chitosan molecules with methoxy poly(ethylene glycol) p-nitrophenyl carbonate to form amphipathic molecular chains, and then self-assembly to form chitosan nanoparticles.
JP 2006241321 discloses a method for producing chitosan nanoparticles which comprises dissolving chitosan in an aqueous acid solution to obtain an aqueous chitosan solution, and adding the aqueous chitosan solution to an aqueous alkali solution, for example, a 3N aqueous solution of sodium hydroxide.
CN 1686560 discloses a method for preparing tetra ammonium salt of chitin which comprises reacting chitosan with cyclopropyl-trimethyl-amine chloride to obtain a quaternary ammonium salt of chitosan, which is mixed with drugs to be encapsulated and sodium tripolyphosphate and then subjected to crosslink to obtain nanoparticles of quaternary ammonium salt of chitosan.
U.S. Pat. No. 4,996,307 discloses a process for preparing water-soluble acylated chitosan which comprises dissolving a water-soluble chitosan having a degree of deacetylation of at least 70% in an aqueous acid solution, diluting the solution with water or a water-miscible solvent such as methanol, and adding an acylation agent such as acetic anhydride to the diluted solution for acetylation. According to the examples of said US patent, an organic solvent, for example, methanol, ethanol or isopropanol, is used as an organic phase.
JP 2000256403 discloses a process for preparing partially acylated chitosan particles which comprises dissolving chitosan in an aqueous acid solution, dispersing the solution in a granulating solvent, agitating the dispersed phase to form particles. The process further comprises forming acylated chitosan particles through acetylation, adding an alkali to the particles and heating the same, partially deacetylating the particles, and then stabilizing them through a crosslinking reaction.
JP 62079201 discloses a process for producing porous granular N-acylated chitosan. In the process, a low-molecular weight chitosan is dissolved in an acidic aqueous solution, and the obtained solution is added to a basic solution to effect the aggregation of the chitosan solution and form the porous granular N-acylated chitosan. The acetylation is performed in an organic phase, for example, methanol or benzene.
CN 1367183 discloses a method for preparing hyaluronic acid-like chitosan which comprises making chitosan undergo an acylation reaction followed by a selective oxidation to obtain a chitosan derivative whose structure is similar to that of hyaluronic acid. The above-mentioned acylation is performed in an organic phase, for example, methanol.
Moreover, the publication entitled “N-Acetylchitosan Gel: A Polyhydrate of Chitin” (Shigehiro Hirano and Ryuji Yamaguchi, BIOPOLYMER, Vol. 15, 1685-1692 (1976)) compares the differences between the gelations resulting from the acetylations of an acetic acid solution of chitosan in methanol phase and in water phase. Said document essentially discusses the gelation resulting from the acetylation of chitosan, and the basic physical properties of gels obtained from said gelation. The physical properties discussed in said document include, for example, in which solvent the gel can be dissolved and the dissolvability of the gel. However, said document only teaches that the addition of a certain amount of acetic anhydride to water phase of chitosan leads to acetylation of chitosan and causes gel formation. The sizes of the resulting colloidal particles are usually too large. Even worse, the particles aggregate and result in a large mass. Chitosan nanoparticles cannot be produced under the operation conditions of said document.
Other relevant publications include: “Physicochemical properties and blood compatibility of acylated chitosan nanoparticles” (Dong-Won Lee et al., Carbohydrate Polymers, 58 (2004) 371-377) and “Reacetylated chitosan microspheres for controlled delivery of anti-microbial agents to the gastric mucosa” (A. Portero et al., MICROCAPSULATION, 19 (2002)), whose contents are incorporated herein for reference.
The known methods for preparing chitosan particles mentioned in the above all have one or more of the following disadvantages. First of all, no chitosan nanoparticles can actually be prepared. Second, the methods involve complicated chemical modification and are time- and labor-consuming. Third, the organic solvents utilized during the manufacture tend to remain in the resulting chitosan particles. Under such circumstances, even if chitosan nanoparticles are produced, whether they are safe for use in medical applications is worrisome.
There is still a need for a method for preparing chitosan nanoparticles which have good biocompatibility and are safe for medical applications.